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mpteach
Elite Laser
Joined: 14 Apr 2006
Posts: 128
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 Posted: 5/04/06, 10:52 PM Post subject: |
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| the photopic resopnse doesnt seem very flat across the range of visible light. Why doesnt sunlight appear greenish? Perhaps sunlight is very blue which would counteract our lack of sensititivity to that region but then what about our lack of sensitivivy to red? |
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thecheat
Wicked Lasers God
Joined: 13 Jan 2006
Posts: 3306
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| Posted: 5/05/06, 8:15 AM Post subject: |
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actually, when you see a rainbow, notice there's less green then red or blue, less blue then red, and so on. it's actually worked out quite nicely!
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stymfalm
Elite Laser
Joined: 05 Jan 2006
Posts: 214
Movie(s): 2
Location: Arizona
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| Posted: 5/05/06, 8:50 AM Post subject: |
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| mpteach wrote: |
| the photopic resopnse doesnt seem very flat across the range of visible light. Why doesnt sunlight appear greenish? Perhaps sunlight is very blue which would counteract our lack of sensititivity to that region but then what about our lack of sensitivivy to red? |
the sun produces white light for all intents and purposes. the reason the sky is blue is because the shorter wavelengths (violet/blue/green) tend to "spread" out much quicker than the longer wavelengths (red/orange/yellow). since there is a concentrated amount of "warm" colors coming straight down, the sun appears yellow. also, at dusk, the sky is red because the shorter wavelengths diffuse too quickly and dont reach you at that angle, but the longer wavelengths do. rainbows are a result of dispersing crystals in the atmosphere in the shape of hexogonal prisms.
hope this cleared things up for ya.
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Warzone
Fusion Laser
Joined: 24 Apr 2006
Posts: 280
Location: Kitchener, Ontario, Canada
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| Posted: 5/05/06, 11:16 AM Post subject: |
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KYLE: Hey Stan, did you see that rainbow this morning?
STAN: Yeah, it was huge!
CARTMAN: Eeh, I hate those things.
KYLE: Nobody hates rainbows!
STAN: Yeah, what's there to hate about rainbows?
CARTMAN: Eeh, you know, you'll just be sitting there, minding your own business, and they're all come marching in and crawling up your leg and start biting the inside of your ass, and you'll be all like "Ay! Get out of my ass, you stupid rainbows!"
STAN: Cartman, what the hell are you talking about?
CARTMAN: I'm talking about rainbows. I hate those frigging things.
KYLE: Rainbows are those little arches of color that show up during a rainstorm.
CARTMAN: Oh, rainbows! Oh yeah; I like those; those are cool.
STAN: What were you talking about?
CARTMAN:Huh? Oh, nothing. Forget it.
KYLE: No, what marches in, crawls up your leg and bites the inside of your ass?!?
CARTMAN: NOTHING!
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Steve0000
Wicked Lasers Master
Joined: 27 Aug 2005
Posts: 573
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stymfalm
Elite Laser
Joined: 05 Jan 2006
Posts: 214
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Location: Arizona
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| Posted: 5/05/06, 4:42 PM Post subject: |
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excellent, the article touches on rods and cones. i also like how it explained that people cant see color in the dark. good article.
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Black Sheep
Wicked Lasers God
Joined: 08 Jan 2006
Posts: 1325
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| Posted: 5/06/06, 12:07 AM Post subject: |
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There was a really big rainbow randomly in front of my house a few days ago when it wasnt even raining...I'll post some pictures when I get back in town (I'm down in Tennessee right now) if this topic is still up here 
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mpteach
Elite Laser
Joined: 14 Apr 2006
Posts: 128
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| Posted: 5/06/06, 12:26 AM Post subject: |
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| no stymfalm, that doeasnt quite answer my question. Why dont green cars look 5 times brighter than red cars? |
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Warzone
Fusion Laser
Joined: 24 Apr 2006
Posts: 280
Location: Kitchener, Ontario, Canada
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| Posted: 5/06/06, 12:54 AM Post subject: |
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I believe it would have something to do that its not red light or green light, its just pigment.
You have white light shining on the red or green pigmant, thus making them closer to the same brightness.
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stymfalm
Elite Laser
Joined: 05 Jan 2006
Posts: 214
Movie(s): 2
Location: Arizona
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| Posted: 5/06/06, 4:53 AM Post subject: |
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| mpteach wrote: |
| no stymfalm, that doeasnt quite answer my question. Why dont green cars look 5 times brighter than red cars? |
well, because there is far less reflectivity in the green car paint so that the two appear the same brightness. in other words, if you had two cars, one with reflectivity of 1 for 500-600nm, and one car with reflectivity of 1 for 600-700nm, the first car would indeed appear to be much brighter. but, if you have the first car's reflectivity changed to say, .1, then it would appear to be close to as bright as the second (red) car.
to answer the question simply, the green cars have less pigment/reflectivity then the red ones.
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retsam resal
Wicked Lasers God
Joined: 01 Apr 2006
Posts: 1017
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| Posted: 5/06/06, 2:12 PM Post subject: |
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The reason why the car isnt brighter there are different dynamics at work. The car isnt really producing its own green light, its reflecting some wavelengths and absorbing others. This is subtractive color mixing. It follows a slightly different set of rules. You couldnt see a car in pitch black dark, but you can sure see a laser.
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thecheat
Wicked Lasers God
Joined: 13 Jan 2006
Posts: 3306
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| Posted: 5/08/06, 2:10 PM Post subject: |
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also, there's less green light in white light then red, further ballancing the mix.
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onlyocelot
4.99mW Green Laser Toy
Joined: 20 Feb 2006
Posts: 3
Location: USA
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| Posted: 5/17/06, 7:33 AM Post subject: rainbows r |
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uhm. Hexagonal crystals cause rainbows? While that may be satisfying to some, the truth is much less new-age.
Water tends to form spherical drops in the atmosphere. Small drops and large drops (up until they become so large that they fall) are all spherical. Light entering the raindrops from the sun are refracted at the . (Yes, this is a simplification: all the light that enters a water drop is refracted.) I mention the ones that enter at the upper surface towards the sun because these are the ones that make the trip to our eyes. They are bent "down" a bit, strike the back of the raindrop at an angle where total internal reflection occurs, and then are refracted again when they leave the drop. The angle of these rays then is just right to reach us on the ground (if and only if the geometries are right). That means sun behind you, droplets in front of you, and up.
The index of refraction is different for different colors. The one number you see in tables of indexes is based on one of the yellow lines of a specific material (the old indexes were based on a yellow line of sodium). However, the indexes vary slightly for each color. This means the angles are different for each color, and that means that, for the small set of rays that make the trip from the sun to your eye, only a small angle of the sky will have the rainbow effect, and subdivisions of that angle will contain different colors, as they are bent less or more.
You don't need a storm to see a rainbow (as has been mentioned). You do need air with water in it, and the right geometries. On the last day of finals last semester, my daughter looked up and saw a rainbow overhead while she was doing her Snoopy Happy Dance. I've seen nested rainbows "on" the surface of clouds around the shadow of an airplane I was flying in. All it takes is a nice spherical drop of water (well, lots of 'em) and the sun and your eyes in the right relation to them.
There's a second, fainter rainbow, usually visible above (outside of) the 'normal' one. This is caused by a double-bounce in the droplets. The light enters at the bottom facing surface of the drop, bounces twice, and exits aimed down at us. We don't get the light bounced from the same drop in 1 and 2 bounce paths, because of the angles, so you're seeing a different set of drops at a different angle. It's less bright because you lose about 8% of the light on each bounce. You can point to which droplets are providing which color in which (primary or secondary) rainbow, because that's where you see that color coming from. (How's that for backwards logic?)
So, no odd crystal theories necessary, no storm necessary, but water droplets and the right geometries are. We don't see three-bounce rainbows (usually) because a) they lose too much light and b) the angle they'd come from is almost directly up, where the sky is usually already brighter from the sun behind you.
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Entity
Wicked Lasers God
Joined: 14 Mar 2006
Posts: 1426
Location: Ca
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| Posted: 5/17/06, 1:56 PM Post subject: |
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I believe only green light is brighter than red light. A red object is no less vissible than a green one. It has nothing to do with varying reflectivity in paint.
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Last edited by Entity on 5/18/06, 12:27 PM; edited 1 time in total |
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Warzone
Fusion Laser
Joined: 24 Apr 2006
Posts: 280
Location: Kitchener, Ontario, Canada
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| Posted: 5/17/06, 9:24 PM Post subject: |
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But this is how colour works.. When white light shines on the cars paint, only one colour reflects back.
The rest are obsorbed. This is why a black shirt will get very hot in the sun as it obsorbes all light. Also why a marker is a lasers best friend.
I dont know where to go from here ...
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